You have to cover the top because hawks or other types of birds of prey will swoop in. I've had this happen already. A hawk found a small opening in the top of my bird netting and got inside the pen. Thank God it didn't kill any chickens!!!
Chickens don't really like oyster shell by itself, in my opinion. Mix it with their feed so they will eat it. Most layer feeds already have enough calcium in it so they do not need extra.
My Father-in law had 6 guineas and they were all killed while free ranging. We caught the culprit..a fox!!! Sometimes fox kill just to kill and leave the carcass for a later time.